A distinction is drawn in optical networks between trunk networks, the so-called “core networks” and access networks. The access networks may be designed to be purely passive and are in this case referred to as passive optical networks (PONs). A PON has the characteristic that a central switching and management unit (“optical line termination”, OLT for short) transmits/receives data to/from a plurality of subscribers. At the subscriber end, a distinction is drawn between optical network terminations (ONT) as the output point for further subscriber-end networks, or optical network terminals (ONU), although this is not relevant in the context of the present invention. Only the term network terminal ONU will still be used in the following text. The connection between the network terminations OLT and the network terminals ONUs is made via at least one optical power splitter or star coupler with the splitting ratio 1:N (N=the number of subscribers). The data accordingly flows in a PON in both directions between the network termination of the higher-level network or the trunk section, and the numerous optical network terminations at the subscriber end.
The signal flow from the OLT unit to the optical network terminals ONUs is referred to as the downstream. The transmission direction from subscriber end ONU to the OLT is referred to as the upstream. Both upstream signals and downstream signals are generally transmitted on a single glass fiber, and this is referred to as the duplex mode.
Different wavelengths are used for this purpose. In the downstream direction, the transmission takes place in a first end “wavelength channel” using the time-division multiplexing broadcast mode, as a continuous data stream. In the upstream direction, the transmission takes place in bursts in a second wavelength channel, using the time-division multiplexing mode. Specific transmission protocols are used to negotiate when an ONU may transmit. The range is currently typically at most 20 km, the splitting factor is at most 1:64, and the data rate is at most 2.5 Gbit/s.
The further development of these systems for higher data rates is defined in various Standards (for example, BPON, EPON, GPON). The latest developments of PONs for data rates of 10 Gbit/s with overall ranges of 100 km and up to 1024 or even 2048 subscribers is referred to as SuperPON. An overview of the current development status of SuperPON is provided in the article “SuperPON—Ein PON der nächsten Generation” [SuperPON—a PON of the next generation], by A. Stadler, M. Rasztovits-Wiech and S. Gianordoli, which appeared in the ITG Specialist Report Volume 189, VDE Verlag, pages 57-62. The high splitting factor considerably increases the attenuation losses of the optical signal in both directions. Attenuations of about 3-3.5 dB can be expected approximately, even with a splitting factor of 1:2. With an overall splitting ratio of 1:512, which corresponds to nine splitting steps, maximum attenuations of 31.5 dB are achieved. If the path loss is also included with this, being about 7 dB, the attenuation losses are correspondingly added. The higher data rate furthermore requires a higher reception power in the optical receivers.
Since the transmission power of the transmitters cannot be increased any further and very high losses must be overcome because of the high splitting factor, the optical signals must be amplified in both directions along the path. This is most easily done by means of an erbium-doped fiber amplifier (EDFA).
EDFAs are normally designed for unidirectional amplification, although they can also be operated bidirectionally. There are various solutions in the literature for the use of EDFAs to provide a bidirectional amplifier. International Patent Application WO 1995/15625 discloses arrangements composed of EDFAs, WDM couplers and optical isolators which ensure bidirectional amplification for optical signals which are propagating in two wavelength channels and in opposite directions. In a first exemplary embodiment, the signals are separated from one another spatially by means of a first coupler, are individually amplified by means of an EDFA in their respective propagation direction, and are then combined again by means of a second coupler. In a further exemplary embodiment, the optical signals are amplified in both directions, in a single amplification fiber. American Patent Application US2004/0228632 also discloses a bidirectional optical amplifier arrangement in which the amplification is carried out for two signals in opposite directions, in one amplification fiber. Two duplex filters are arranged within the WDM system and are used for combination or for separation of the signals at the transmission or reception end. The filters each have two connections at the transmitter and receiver ends. At the other ends, the filters each have a common connection for the signals in the opposite direction, via which the filters are connected via a filter which is doped with erbium.
In addition to the basic amplifier design, more stringent requirements exist within the PON systems for the amplifier control for the upstream signals which are transmitted in bursts, because relatively long time gaps, for example of several hundred ∥s, can occur between individual data bursts, and because the data bursts may have amplitude differences of up to 17 dB. The characteristic saturation and recovery time constants of EDFAs are likewise in the range between 100 μs and 10 ms, so that power fluctuations of the input signal lead to transient processes in the output signal of the EDFA. Gain control systems are normally used to set a constant gain, in order to keep the inversion within the doping element erbium at a predetermined nominal level. If no signal is applied to the amplifier, gain control is in principle impossible, since there is no reference signal at the input of the amplifier. When signal power is now applied again to the EDFA input as a result of the arrival of a data burst, then the pump power is initially not optimally set in order to maintain a constant gain. A time period has to pass before the nominal value of the gain and output power are reached. This major dependency of the amplifier dynamic response on the input power is intended to be suppressed for the amplification of the burst-like upstream data signals.
A search is therefore being carried out for simple solutions for an optical amplifier arrangement, in order to achieve amplification which is as optimum as possible for the continuous downstream signal and the upstream signal, which is in the form of bursts.